In medical practice, imaging systems play an important role in the examination of patients. The representations of the patient's internal organs and structures generated by imaging systems are used to diagnose diseases, to plan operations, to carry out operations or to prepare for therapeutic measures. Besides systems which generate the patient image data by applying magnetic fields (for example magnetic resonance tomographs) or with the aid of ultrasound waves, imaging systems which use X-radiation are particularly important. These systems include for example angiography devices, mammography devices and special computer tomography systems.
In a typical arrangement of an imaging system based on X-radiation, an X-ray source lies on one side of a patient and an X-radiation detector lies on the other side of the patient. The detector is used to determine how strongly the X-radiation emitted by the source is attenuated when it passes through the patient's exposed body region. With the aid of the spatial dependency of this attenuation, image data can be generated.
A typical X-radiation detector converts the strength of the X-ray beam into an electrical signal, which can be fed to an image processing system. Conventional embodiments of a corresponding detector are radiation converters, for example so-called scintillation crystals with a downstream image recorder, for example a photodiode array.
In the scintillation crystal, the incident X-radiation causes the creation of light pulses owing to the photoeffect. The generated light pulses are then converted by the photodiode array into electrical signals, which carry information about the strength of the X-radiation. For example, the amplitude of an analog electrical signal might describe the strength of the X-radiation. Furthermore, detectors with integrated analog-digital converters, which output digital electrical signals to describe the radiation strength, are now also customary.
X-radiation detectors are conventionally supplemented with so-called collimators or collimator plates, which are aligned in the direction of the X-radiation. Scattered rays, which deviate from this direction, are absorbed by the collimator plates. Image artifacts, which result from scattering, can thereby be reduced. Such collimator plates are, for example, made from lead or tungsten.
A fundamental problem with X-radiation detectors is that the individual components generate heat during operation of the detector, which can compromise the function of the detector per se or even prevent it from functioning. For example, the accuracy of the analog-digital conversion can deteriorate at operating temperatures which are too high. Conventional maximum tolerable temperatures are about 60° C. in this case.
This fundamental problem has repercussions especially in computer tomography systems. In order to reduce image recording times and increase the image resolution in computer tomography systems, detector arrays are used with a multiplicity of radiation detectors which are often operated simultaneously. In current systems, a heat loss with a power of 1000 W can readily occur in the detectors, which without further measures could greatly compromise or prevent operation of the detectors. Accordingly, it is necessary to cool the X-ray detectors during operation.
The use of conventional cooling systems, for example cooling with the aid of a coolant and a heat exchanger, however, is elaborate and uneconomical for example for a computer tomography system owing to the large number of detectors. Furthermore, many computer tomography systems use detectors mounted in a mobile fashion, which surround the patient in the shape of a circle. This makes cooling with a liquid even more difficult.
Another already existing embodiment of detector cooling in computer tomography systems uses fans, which create an air flow on the other side of the detector from the X-ray source. The cooling power achievable in this way, however, is insufficient for larger detector arrangements since the thermal conduction resistances from the heat sources in the detector are too great.